1. Field of the Invention
This invention relates to a method of making packaged grid array assemblies including integrated circuit chips or dies. More particularly, the invention relates to a method which yields a ball grid array incorporating a flexible circuit substrate which can be manufactured reliably and at a low cost.
2. Related Art
As integrated circuits have become more complex, a need has arisen for a packaged integrated circuit having a large number of high density, reliable external package connections It is also desirable to include in a packaged integrated circuit one or more conductive layers for signal routing and provision for ground and power metallization traces. To meet these needs, the ball grid array has been developed as illustrated in FIG. 1 of the commonly assigned related application Ser. No. 08/214,339 filed Mar. 16, 1994 in the name of Bruce J. Freyman et al.
A typical prior art ball grid array includes a semiconductor die, referred to herein as an integrated circuit (IC) or chip, attached to a die attach surface on a printed circuit board (PCB) with adhesive. Electrically conductive bond pads on the die are connected by electrically conductive bond wires to electrically conductive traces and/or electrically conductive regions formed on or adjacent to the die attach surface of the PCB substrate. Electrically conductive vias are then formed through the substrate from the traces and/or regions on the die attach surface to a mounting surface of the PCB substrate which is opposite the die attach surface. The electrically conductive traces formed on the mounting surface extend to solder pads formed on the mounting surface. Solder bumps are formed on each of the solder pads. The solder bumps are later reflowed to attach the BGA package to another electronic component such as a PCB mother board. An encapsulant such as plastic is formed to enclose the semiconductor die, the bond wires, and a portion of the die attach surface of the substrate, including most of the traces and/or regions. The vias are shown outside the encapsulant in FIG. 1 of the commonly assigned application Ser. No. 08/214,339, however, in other embodiments the vias can be inside the encapsulant. The encapsulant is frequently formed by injection or transfer molding or by other well known molding methods and equipment to form the encapsulant over the die in a ball grid array. Liquid (glop top) encapsulant also can be used.
Currently, molded plastic ball grid array packages are the least expensive method of producing a ball grid array style semiconductor package. In the prior art, the most expensive component of the ball grid array package is the substrate, which as discussed above, is typically a laminated glass fiber material such as printed circuit board (PCB).
In an effort to reduce the cost of plastic ball grid array packages, a good deal of research has been directed towards investigating alternative substrate materials to replace the relatively expensive PCB substrates. One promising material for replacing PCB substrates is a polyimide film with metal traces and/or bonding positions formed on the polyimide film. The combination of polyimide film with metal traces is referred to in the art as a flexible circuit or flex circuitry. The metal traces of the flex circuitry are typically formed by either applying a laminated copper foil to the polyimide film and etching to define the conductors (subtractive process) or by vacuum depositing copper directly onto the polyimide film followed by pattern plating (additive process).
Flex circuitry is typically fabricated into a ball grid array substrate by two different approaches. In the first approach, the flex circuitry is fabricated into a ball grid array substrate by pre-applying the metal, such as copper, on both sides of the polyimide film. Once the metal is deposited, standard drilling, punching, or etching techniques, well known to those of skill in the art, are used to form vias through the flexible circuitry. Once the vias are formed, standard photo-delineation techniques are employed to produce the desired metallization pattern, including metal traces and bonding locations. A flex circuitry substrate created by this first approach is identical in format to the printed circuit board substrates currently used with plastic ball grid arrays. However, there is the critical difference in that the circuits and traces are formed on the flexible polyimide film rather than a glass-reinforced printed circuit board laminate.
Using this first approach of forming a flex circuitry substrate, the upper surface of the substrate contains a metal pattern that is used for mounting the die and wire bonding the die to the substrate. The electrically conductive vias conduct the electrical signals from the upper surface of the flex circuitry substrate to the circuit pattern on the lower surface of the flex circuitry substrate. The circuit pattern on the bottom of the flex circuitry substrate carries the electrical signals from the via to an array of metal terminations, usually solder balls, which are attached to pre-determined locations on the bottom of the flex circuitry substrate.
In the second approach to using flex circuitry to form a ball grid array substrate, the flex circuitry is formed with the metal circuit and trace patterns formed only on the upper surface of the flexible polyimide film. Using this second approach, the metal pattern formed only on the upper surface of the flexible film is used for chip mounting and wire bonding the chip to the circuit. The circuit pattern typically includes an array of round copper features outside of the wire bond pad. The backside of the copper film on these round metal features is then exposed by holes in the flexible film which can be formed by etching, drawing, punching, or other techniques well known to those of skill in the art.
Using this second approach of forming a flex circuitry substrate, the back side of the copper film is then used as a surface for the attachment of solder balls during the assembly of the plastic ball grid array package.
One advantage of forming a plastic ball grid array package using a flex circuitry substrate is that the flex circuitry substrate can be made in a reel-to-reel format in very high volume. Consequently, flex circuitry substrates have the potential to be a lower cost substrate then the printed circuit board substrates in current use. Another advantage of flex circuits is that they offer the capability to produce finer metal conductor patterns. For flex circuits that use an additive process to form the conductors, wire bonding fingers can be placed more closely together compared to conventional subtractive printed circuit board substrates. This allows more interconnections to the IC chip per unit area, which reduces cost.
Unfortunately, there are several problems associated with using flex circuitry substrates for plastic ball grid array manufacturing. First of all, the flex circuitry is very difficult to handle in a factory environment, i.e. in a mass production environment, because flex circuitry substrates lack the rigidity necessary for automated manipulation in mass production assembly lines.
In addition, the flex circuitry substrates that are formed using the first approach described above, i.e. the flex circuitry substrates that have metal traces on both surfaces and are molded in the same format as printed circuit board substrates, have a high warpage factor and exhibit poor solder ball co-planarity. This is because flex circuitry substrates typically warp during the curing of the mold compound used to encapsulate the ball grid array. This problem is particularly significant with newer generations of chips because the number of elements present on newer integrated circuit chips requires more connections between the integrated circuit chip package and the mother board and, as a result of the increased number of necessary connections, it is becoming more and more important that the solder balls making the external connection have a high degree of co-planarity, i.e., the solder balls all have the same height. In the prior art, the problems of warpage and decreased co-planarity were avoided because standard, i.e. printed circuit board, substrates have the requisite rigidity and have relatively low warpage levels. Consequently, the same flexibility that allows flex circuitry to be made in a reel-to-reel format causes warping problems further along in the processing.
The warping and special handling/processing problems associated with flex circuitry ball grid array substrates has prevented this highly advantageous substrate production technique from gaining widespread acceptance in the packaging industry.
What is needed is a method for creating plastic ball grid arrays incorporating flex circuitry substrates that allows a ball grid array package with a flex circuitry substrate to be manufactured using standard manufacturing equipment and techniques known in the industry, and yet, does not exhibit the high warpage and poor solder ball co-planarity problems currently associated with flex circuitry substrates.